1. Field of the Invention
The present invention relates to a cyclometalated transition metal complex and an organic electroluminescent device using the same, and more particularly, to a cyclometalated transition metal complex that can emit light ranging from a blue wavelength region to a red wavelength region through triplet metal-to-ligand charge-transfer (MLCT) and an organic electroluminescent device including an organic layer composed of the cyclometalated transition metal complex.
2. Description of the Related Art
Organic electroluminescent (EL) devices, which are active display devices, use the recombination of electrons and holes occurring in a fluorescent or phosphorescent organic layer when current is applied to emit light. Organic EL devices are lightweight, have wide viewing angles, produce high-quality images, and can be manufactured using simple processes. Organic EL devices can produce moving images with high color purity with low consumption power and low voltage. Accordingly, organic EL devices are suitable for portable electronic applications.
In general, an organic EL device includes an anode, a hole transport layer, an emission layer, an electron transport layer, and a cathode stacked sequentially on a substrate. The hole transport layer, the emission layer, and the electron transport layer are organic layers. The organic EL device may operate thorough the following mechanism. First, voltage is provided between the anode and the cathode. Holes injected from the anode move to the emission layer through the hole transport layer, and electrons injected from the cathode move to the emission layer through the electron transport layer. In the emission layer, the holes and electrons are recombined, thus producing excitons. The excitons decay radiatively, emitting light corresponding to the band gap of a material.
Materials for forming an emission layer are divided into fluorescent materials using singlet-state excitons and phosphorescent materials using triplet-state excitons, according to emission mechanism. The fluorescent material or phosphorescent material may form the emission layer, or a fluorescent or phosphorescent material-doped host material may form the emission layer. When electrons are excited, singlet excitons and triplet excitons are generated in a stastics ratio of 1:3.
When an emission layer is composed of the fluorescent material, triplet excitons that are generated in the host cannot be used. On the other hand, when an emission layer is composed of the phosphorescent material, both singlet excitons and triplet excitons can be used, and thus, an inner quantum efficiency of 100% can be obtained (see Baldo et al., Nature, Vol. 395, 151-154, 1998). Accordingly, the use of phosphorescent material results in higher luminance efficiency than the fluorescent material.
When an organic molecule contains a heavy metal, such as Ir, Pt, Rh, or Pd, spin-orbital coupling occurs due to a heavy atom effect, and thus, singlet states and triplet states are mixed, allowing a forbidden transition to occur and thus effectively emitting phosphorescent light even at room temperature.
Recently, highly efficient green and red emissive materials that use phosphorescence having the inner quantum efficiency of 100% have been developed.
For example, transition metal compounds that include a transition metal such as Ir or Pt have been developed. However, materials that are suitable for highly effective full-color display and low power consumption fluorescent applications are green and red emissive materials only. In other words, blue phosphorescent emissive materials have not been developed. Accordingly, a phosphorescent full-color device cannot be manufactured.
In order to resolve this problem, blue emissive materials (disclosed in WO 02/15645 A1 and US 2002/0064681 A1); cyclometalated transition metal complexes that contain a bulky functional group that can deform the molecular geometry for widening the gap between a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) or a functional group with a high ligand field such as a cyano group (disclosed in Mat. Res. Soc. Symp. Proc. 708, 119, 2002; and 3rd Chitose International Forum on Photonics Science and Technology, Chitose, Japan, 6-8 Oct. 2002. ); an Ir complex, such as Ir(ppy)2P(ph)3Y where Y═Cl or CN (disclosed in US 2002/0182441 A1); and an Ir (III) complex containing a cyclometalating ligand, chelating diphosphine, Cl, and a cyano group (disclosed in US 2002/0048689 Al) have been developed.
In addition, US Patent Publication No. 2002-0134984 discloses a cyclometalated transition metal complex containing nitrogen and carbon and an organic EL device using the same.
However, none of these blue emissive materials simultaneously have desired color purities, luminance efficiencies, and lifetimes.